Faults in electrical power systems cannot be avoided. Fault currents flowing from the sources to the location of the fault lead to high dynamical and thermal stresses being imposed on equipment e.g. overhead lines, cables, transformers and switchgears. Today's circuit-breaker technology does not provide a full solution to selectively interrupting currents associated with such faults [1].
The growth in electric energy generation and consumption and the increased interconnection between networks lead to increasing levels of fault currents. In particular, the continuous growth of electrical energy generation has the consequence that networks reach or even exceed their limits with respect to the short current withstand capability. Therefore, there is considerable interest in devices, which are capable of limiting fault currents. The use of fault current limiters (FCL) allows equipment to remain in service even if the prospective fault current exceeds its rated peak and short-time withstand current. Thus, replacement of equipment (including circuit-breakers) can be avoided or postponed to a later time. Moreover, the use of FCLs already in the design stage takes advantage of using lower cost equipment: e.g. transformers with lower impedance, cables with reduced cross-sections, circuit-breakers with lower current interruption capability etc. [1].
Sometimes the problem of the fault-current limiting may be resolved by inserting current limiting reactors (CLR) with constant inductance as seen e.g. in [2] for high voltage and in U.S. Pat. No. 7,330,096 [3] for low voltage cases. However, additional inductance may result in an undesired voltage drop and in a decrease in system stability and reliability. Therefore, a variable impedance device that changes from small negligible impedance at nominal current to high impedance at fault conditions is a most desirable solution for the fault-current limiting problem.
One of the most attractive principles for FCL realization with variable impedance is the saturated core FCL [4]. FIG. 1 illustrates schematically its principle of operation. An FCL 10 comprises two ferromagnetic cores 11a, 11b, which are kept in saturation during normal (i.e. non-fault) operation by the magnetic field 25a, 25b generated by two bias coils 13a, 13b fed from a DC supply 24. Two AC coils 12a, 12b are wound around these two cores and arranged in such a way that their field directions 26a, 26b for each half cycle of AC current are in opposite directions with respect to the bias field 25a, 25b (i.e. in the first half cycle, field direction 26b is opposite to the direction 25b in core 11b, and during the second half cycle 26a is opposite to the direction 25a in the core 11a). Thus, AC coils 12a, 12b, connected in series with AC source 21 and load 22, exhibit low impedance under normal (i.e. non-fault) conditions. In case of an overcurrent, the increased AC current in the AC coils drives the core (according to the sign of the AC current 11a or 11b) out of saturation and the impedance of the FCL increases. In short-circuit conditions, the limited fault current triggers the opening of a circuit-breaker 23 and maintain a proper operation of the AC system 20.
A major drawback of known FCLs is their large mass and volume [1] and early attempts for reducing the mass were proposed [5] more than 40 years ago. However, very high requirements for DC ampére-turns limited use of these devices to large power applications.
This limitation was to a great extent removed with development of super-conductivity applications. U.S. Pat. No. 4,045,823 to K. C. Parton et al. [6] discloses a current limiter for a power alternating current system. The current limiter has for each phase a pair of saturable reactors whose coils are wound in opposite directions relative to superconducting bias coils.
An example of a single phase FCL for medium voltage is described in [7]. From data revealed in this work it can be derived that the mean incremental permeability in a saturated core was about 1.6 and the magnetic field strength caused by the DC bias coil was more than 1000 Oersted. In [6] and [7] it is noted that the current-limiting level is matched to a specific current supplied to the bias coil. Thus, in cases of fault current levels lower than the designed level, it is possible that the FCL will react to provide insufficient impedance.
Interest in the saturated core FCL has been spurred by the development of high temperature superconductivity (HTS) applications such as [1, 4]. In [8] a three phase device with six cores and one DC superconductive bias coil is described. The transformer coupling between the AC coils and the DC bias coils, which causes low limiting capability for three phase symmetrical fault currents as well as influence on the DC supply in unbalanced load/fault conditions and in one and two phase fault currents, is another major disadvantage of known saturated core FCLs. Patents [5, 6] overcome this problem by adding additional inductances in series with a DC bias coil, but in doing so the first disadvantage of high mass is even further exacerbated.
FIG. 2 shows schematically a different known approach for a FCL 30 with saturated core as disclosed in [10]. The FCL 30, described therein, comprises a ferromagnetic core 31 having two first (“long”) limbs 33a, 33b and two second (“short”) limbs 32a, 32b. An AC coil 35 is wound around the two first limbs 33a, 33b such that AC current causes flux in one direction in both limbs in each half cycle. Two superconducting DC bias coils 34a, 34b surround limbs 32a and 32b thus providing flux in the first limbs 33a, 33b in opposite directions and causing saturation of the core 31 in the normal (i.e. non-fault) state. By such means, there is provided a closed magnetic circuit for the bias field and an open magnetic circuit for the AC field. The bias coils and their DC supply are arranged in such a way to provide a deep controllable saturation in normal conditions and also provide the possibility to reduce or even cancel DC current in fault conditions. Thus under normal conditions and in a wide range of the acceptable overload state desired low impedance may be provided by DC current changing. In fault current conditions during each half cycle, one of the limbs 33a or 33b is forced out of saturation. Thus increasing the impedance of the AC coil 35 causes an instantaneous increase of the voltage drop and fault current limiting. At the same time by increasing the voltage drop, a control signal may be provided for reducing or canceling DC current. It should be noted that an open magnetic circuit for the AC magnetic field provides a wide range of the fault-limiting level without the need for changing the DC bias field level.
FIG. 3 shows a similar principle as described in JP 2002 118956 [11], which discloses a current limiter that includes a pair of first and second magnetic cores 2a, 2b facing each other and an AC coil 3 wound around the cores 2a, 2b. However, in this case the cores 2a, 2b are maintained in saturation state at normal conditions by two permanent magnets 1a, 1b. The ferrous parts of this construction appear as an open magnetic circuit for flux caused by AC current and as a closed magnetic circuit for flux caused by permanent magnets 1a, 1b. In this respect, the FCL shown in FIG. 3 is the same in principle as the FCL depicted in FIG. 2, but unlike it, there is no possibility to vary the impedance in the normal state and an additional disadvantage of a FCL with permanent magnets is thermal stress at least in fault conditions, which can increase the required recovery time, similar to resistive superconducting FCL devices [1, 4].
The saturated cores of the FCL as described in [5,10,11] are suitable for only a single phase of a 3-phase supply. Thus practical 3-phase AC systems require three such ferromagnetic structures, thus resulting in a massive construction.
WO 2010/056122 [12] published after the priority date of the present application, discloses a 3-phase current limiter wherein three AC coils for each of the respective phases are wound on the same core. The AC coils (termed “flux generators”) are all three wound in the same direction with respect to each other (e.g. clockwise or counterclockwise, with respect to a coil axis). It is stated that this yields the particularly great advantage in the use of the three-phase current that in case of fully balanced currents (of the three phases) the magnetic fluxes generated by the three AC coils can cancel each other out completely.
While this is true, this prevents the core from being de-saturated in a balanced fault current event, as is required for limiting the fault currents and so the current limiter taught by [12] appears to be incapable of operation for balanced fault current events.